1. Field of the Invention
The present invention relates to an apparatus for measuring the luminous lifetime of a sample. More particularly, the present invention relates to an apparatus for measuring a radiation lifetime which is not as long as the pulse width of an emitted exciting light.
2. Description of the Prior Art
When the luminous lifetime of a sample is sufficiently longer than the pulse width of the emitted exciting light, only the luminous shape thereof is measured and the luminous lifetime thereof is derived from the attenuation curve of said luminous shape.
When the luminous lifetime of a sample is not as long as the pulse width of the emitted exciting light, the luminous shape thereof is influenced by the pulse width to give rise to distortions.
As a result, in the second case, it is necessary to measure not only the luminous shape of a sample but also the pulse shape of the emitted exciting light. These quantities are expressed by the true luminous shape and the convolution integral equation. If the measured luminous shape is I(t), and the pulse shape of the emitted exciting light is P(t), and the true luminous shape is G(t), then the convolution integral equation is expressed by the following formula: EQU I(t)=.sub.0 .sup.t P(t')G(t-t')dt'
G(t) is obtained from I(t) and P(t) by a deconvolution operation to find the luminous lifetime.
In a case when the luminous lifetime of the sample is measured by conventional methods on the basis of a time correlation photon counting technique, a single photoelectron pulse can be measured for one radiation time interval and the measurement requires a long time period due to a low photometric efficiency.
An apparatus for measuring a plurality of photoelectron pulses for one radiation time interval in order to eliminate the above described defects is disclosed in Japanese Laid-Open Pat. No. 137843/1982. This apparatus, as schematically shown in FIG. 1, consists of a light source 1 which emits a pulse of light, an optical detector 3 which receives light radiated from a sample 2 to which the pulse of light is supplied and which emits a photoelectron pulse, a start pulse generating means 4 (usually comprising a photomultiplier tube 5 and a waveshape regulating circuit 6) which emits a start pulse when the pulse of light is radiated from the light source 1, a plurality of time-voltage converters 7 (hereinafter referred to as TACs) which are activated by a start pulse emitted from the start pulse generating means 4 and which are stopped in turn by a plurality of photoelectron pulses which are used as stop pulses and which are emitted from the optical detector 3 through a pulse shape regulating circuit 6', a multiplexer 8 which selectively outputs signals from the TACs 7, and a memory circuit means 9 which stores output signals from the multiplexer 8.
When the luminous lifetime of a sample is measured by a deconvolution operation in a conventional apparatus, the sample 2 must be replaced by a scattering body at those times other than the time when the luminous shape I(t) of the sample 2 is being measured so as to measure the pulse shape P(t) of the emitted exciting light. Even though a multi-channel TAC is generally used in order to shorten the measurement time, the resolution power of this apparatus for pulses is limited. In a case when a plurality of photoelectron pulses are generated within this limit, an individual photoelectron pulse cannot be distinguished from another pulse. That is to say, the effect of a multi-channel TAC cannot be completely exploited. Consequently, the defect that the measurement requires a long time period still remains. Accordingly, if a sample has a comparatively short lifetime, even though the pulse-exciting light frequency is about 20 KHz, it frequently takes one hour to measure the pulse shape of the exciting light and one or more hours to measure the luminous shape, that is to say, two or more hours in all. In some cases, when a lower frequency pulse laser is used, it takes 10 or more hours to measure only the pulse shape of the exciting light. In addition, in some cases when the measurement requires such a long time period, the pulse shape of the exciting light may change. Accordingly, in a case when P(t) and I(t) are measured at different times, the measured P(t) may be different from P(t) at the time when I(t) was measured. As a result, even though the luminous lifetime is obtained by a deconvolution operation, a large error is produced.